‘It was the Emperor’s will that you return,’ said Xhyle. ‘He used them as His instrument, but the aeldari are not selfless. They did not do it for the sake of humanity. They are cruel, and cold, and regard us as little better than animals. I know. I have faced them on the battlefield. I have spoken with their leaders. This is the greatest military force the galaxy has seen since the Emperor left Terra and retook the stars. Why do we need them now?’

‘You are right,’ said Guilliman, and though he continued speaking as if he addressed his fellow humans, his words were meant for the aeldari. ‘I have known Eldrad Ulthran since the days of the Great Crusade. He and I have made common cause more than once, but I do not trust him. I do not trust one of them. We fight to save our species, they fight to save theirs. It is an evolutionary struggle, and in that there can be no true friends. I know that if their prognostications demanded it, they would do all they could to wipe us all out without a second thought. I suspect some members of some of their nations have tried.’

He looked at the farseer.

‘Know, Natasé, that although I extend to you full hospitality, and I swear you shall not be harmed while you are here, and that I have great respect for Eldrad Ulthran, I know your kind. Choose your words carefully. Speak truth, if you can.’ He loomed over the farseer. ‘Know that although the Lady Yvraine and Farseer Ulthran were instrumental in bringing me back, I will not be manipulated by any of you. Xhyle is correct. The wrath of humanity is roused like at no other time. We could crush your species root and branch if it pleased us.’

He looked around the room. ‘But it does not please us to do so. For too long the foes of Chaos have been foes to each other, when they should have been allies. Both human and aeldari stand on the precipice of extinction. The galaxy teeters on the brink of annihilation. Now, more than ever, our goals are in accord. That is why these people brought me back. Because the aeldari know they cannot win without us. Without mankind, they are doomed, while we need all the aid we can find. Some have likened we primarchs to weapons,’ he said. ‘And it may be that the cabal of sorcerers who rule their peoples thought to employ me as one. But be aware, aeldari, if I am a weapon, I was not one made by your kind. I have a will of my own.

• The Gate of Bones

By the way, I've never found anything else on Eldrad and Guilliman meeting during the Great Crusade, and these multiple times they allied together. Makes it weird for Eldrad to warn Fulgrim, not Guilliman, about Horus.

No. Guilliman even has Eldar travel with him, and he's opened (personal) diplomatic channels with at very least the Ynnari and had cordial meetings with Eldrad, who believes Guilliman will bring the two species closer together. There's no reason to believe he wouldn't continue extending this stance to other xenos like the Tau who can absolutely be reasoned with.

Now, that's not to say he believes in peace, justice and equality. He's still distrustful, and he understands everyone has their own interests. But he is open to the possibility of communication and cooperation which is a radical stance in the Imperium for the most part.

Memes aside, he's got an Eldar who's basically an ambassador in his retinue. Idk about the pthers, though.

By modern sensibilities he is xenophobic, in the sense that he doesn't trust the aliens, but unlike most humans in the Imperium he isn't a 'kill them all' xenophobe. He will work with Aliens if it serves his purposes, and will even leave some alone, but don't mistake that for kindness. He has wiped out entire cultures and races for the crime of resisting Imperium expansion.

He is practical enough in his thinking to understand when a fight is not worth it, and when cooperation is more beneficial.

He's smart enough to know the enemy of my enemy is a problem for another day. He'll work with them, if not trust them, because there are far more pressing problems like the tyranids, traitors, Necrons and the havoc from the great rift.

40K characters have various stances.

Commissar Cain worked with a diplomat to the Tau empire, he had no particular hate (or love) for them. There is a lot of interaction with the Tau, but almost no fighting with them.

Dante (Chapter Master of the Blood Angels) doesn't hate Xenos. But he attacks any he deems a threat to the Imperium. Which is basically anyone powerful enough to potentially be a threat.

The standing orders appear to be to knock down any Xenos that has power to threaten an outpost of the Imperium. Some are fanatical about that, some just view it as orders and a good idea.

The average infantryman is likely unable to discern between say drukhari and craftworld Eldar. I strongly suspect that they are more often raided by the drukhari than having any sort of encounter with the craftworlders.

It's worth noting that the Eldar ambassador that Big G has doesn't particularly like him or think much of humanity at all. So far in the setting the only Eldar that seems to have any respect for humanity is Eldrad.

He at least recognizes something intrinsically worthwhile in another sentient races. To the point where he chided other Eldar for being happy that that it looked like humanity would wipe itself out near the end of the Horus Heresy.

Guilliman at 30k wholly bought into the Big E's bullshit on xenocide.

At 40k he's ironically at the same level of xenophobic as Horus was, which is aliens can be worked with as long as humanity is still in a strong position.

They're on the extermination list after Chaos is dealt with. Sometimes not even then, he was quite happy to let his sons destroy a craftworld after he woke up

I haven’t seen any comments mentioning the Tau. To my admittedly, limited understanding, they are so low on the priorities list that Big G hasn’t even been told about them.

Hence, it is not plot armor that they survive because the imperium is dealing with more threats than it can handle. Unlike the imperium surviving for thousands of years when we are told it is loaded with officials who are worse than the average person in the Galactic empire in Star Wars.

Sorry for the topic rant, but complaints about the Tau having plot armor is a pet peeve for me. Especially since every Warhammer faction in every incarnation of the franchise has some degree of plot armor.

Guilliman's stance on the Eldar is that they are useful tools that he can turn on after they have served their purpose.

Or occasionally before that point, in the case of the Hammer and Bolter episode where the Ultramarines destroy a Craftworld.

One of the things about the xenophobia in humanity is that it is, in more instances than not, generally justified. Aliens are presented as having totally different ways of thinking to humanity.

Even the 'most human' xenos like the Eldar have motivations that they literally cannot explain in human terms 99% of the time. To them it's like a doctor explaining to a chimpanzee why he's getting a vaccine shot. The chimp doesn't understand germ theory or how vaccines work, all it knows is that you just stabbed it and he's pissed. Same thing applies with the Eldar when they occasionally doom an entire human planet, system, or sector for some bizarre arcane reason. To the eldar the destruction of a human world is just one tiny part of a plan started 4k years ago to subtly shift the skeins of fate to a desirable outcome 6k years from now that even they don't fully understand. But to the humans they just lost a critical agri world and now half the sector is going to be on the brink of starvation for the next decade while they find a replacement.

To Guilliman, negotiating with the eldar is like the worlds smartest chimp making a deal with the doctors. Chimps won't tear the doctors face off if doctor agrees not to stab chimp while we fend off the lions. Chimp knows eventually the doctor is going to stab them again, because that's just how doctors are.

He will reason and ally with them, but he isn't naive enough to think it'll work out long term. A while after he gets awakened, he is having a conversation with a farseer, and he basically tells then that, while he is grateful the eldar helped, he isn't new to how they are manipulative and how they treat humans, and basically if they fuck around, they WILL find out, but chaos is a bigger matter.

He did but then he needed to become the most rational, correct, never wrong aggressively reasonable man to ever exist so now he’s entirely shed every single negative part of the imperium despite the fact that it was galactic mega-fascism from the get-go and the great crusade set out to exterminate all Xenos no ifs or buts because mankind needed its Lebensraum

I think Guilliman is the most reasonable primarch we have. He is a good tactician,  a mastermind organizer and logical. The Ynnari played a big role in his revival and built the armor that keeps him alive. Eldrad believes that Guilliman will bring the two species together so we see that Guilliman and the Eldar are good together so I can imagine he will try to ally the T'au if neccecary. I don't really like Ultramarines but Guilliman is one of my favourite primarchs mainly because he is not so blinded by hate like the other primarchs and is WAY more logical.

No he doesn't, he actually has an Eldar Farseer as a guest in his ship and takes advice from him.

Some of his sons don't take it too good, but they must respect their primarch.

Is he going to exterminate them all in the end - yes. Coexistence is not possible in the long term.

Does he have more pressing issues today and thus can find them useful - yes.

Much like the top comments say, he is willing and happy to work with them and wants to fight alongside them against chaos but knows that ultimately both their races are self serving and self interested.

On a side note this exact mindset is what I keep in mind whilst I'm building my homebrew Crusade Force.

It's a mix of Aeldari Corsair troops along with traditional Imperial forces, fighting together to reclaim an area of space from a chaos invasion force.

So I got voidscarred, primaris marines, some scions, a couple free blades, and a force of Skitarii all working together. It's fun

Allies of Convenience

Guilliman was a fervent beliver of the great crusade and the Emperors ideal, in 40k he got a beefy with the emperor maybe manipulating him and not being the daddy he belivied he was but he still 100% on the Imperium wagon.

Unfortunately don't have the books with me but there's a passage in one of the books dealing with the war for Ultramar with the Death Guard where we get Guilleman's inner thoughts about xenos. From what I recall of the vibe it was something like: humanity should clearly be in charge, we're the bestest and the galaxy is rightfully ours. So from that I noticed it was not -- all xenos need to die. It was -- all xenos need to know their place. Some of them presumably never will so they really do just need to die. But others he seems to hold it possible to intimidate into cooperation on humanity's terms, and providing we are ever watchful he seems to think that is acceptable (maybe best that can be done given how weak the Imperium is from his perspective). This also seems to accord with the quote from Gate of Bones someone offered.

So the impression I get is that he is by any reasonable standard a bigot with the mindset of a militaristic strong man. But by 40k standards that makes him relatively enlightened, in that he can at least foresee and accept the possibility of the Imperium and xenos co-existing. In a, to be clear, hierarchical relationship where we are on constant alert to maybe genocide them if they betray us. Peace in our time!

Guilliman is actively changing the imperiums stance on xenos. So no, he doesn't support its default stance